Design Parameters for Slab-on-Grade Foundations

Design of slab-on-grade foundations on expansive soils requires that the geotechnical engineer determine the maximum differential heave and the distance in from the edge of the slab over which the differential heave takes place (PTI, 2004). Currently the most commonly used method relies upon semi-empirical methodologies developed using observations primarily in the Southwestern United States. This methodology utilizes broad climatological data based on the Thornthwaite Moisture Index. Heave prediction methodology is based on the suction compression index and equilibrium suction states. This paper takes a different approach and utilizes the actual climatological and irrigation data. Oedometer methodology is used for heave prediction. Seepage analyses are conducted to evaluate wetting patterns over time beneath slab-on-ground along different vertical planes. The manner in which differential heave takes place over time and the pattern of heave development from edge-heave to center-heave is investigated. Maximum differential heave and edge moisture variation distance are plotted as a function of time. It is shown that in arid and semi-arid environments, as irrigation is applied around the structure, edge-heave initiates and governs over the life of the structure. This pattern, however, will change due to accidents such as broken pipes or wetting from other sources. The maximum differential heave and the edge moisture variation distance computed for sites with various drainage conditions using the methodology presented in this paper could be significant higher than those calculated using the PTI method.